The present invention is related to optical networks in general and, in particular, to OFDM transponder interfaces.
A present trend in telecommunications is the installation of optical networks with ever increasing data transfer rates. Optical networks in general allow high signal bandwidths, and multiple communications channels can be created over a single optical fiber with techniques, such as WDM (Wavelength Division Multiplexing) and its successor DWDM (Dense Wavelength Division Multiplexing). (Hereafter, for brevity's sake, any technology which uses wavelength to define a path in an optical network is simply referred to as WDM.) Current and proposed optical networks offer bandwidths of 10, 40, and even 100 Gbs (Gigabits per second). This rise in bit transfer rates is a consequence of not only technical development but also the burgeoning demand for the much anticipated applications of voice, computer data and video delivery with optical networks. For example, such applications include video-on-demand, such as Internet TV, amateur video blogs, and social networking video.
Among the different techniques which are currently being investigated to increase the optical performance of optical networks is OFDM (Orthogonal Frequency Division Multiplexing). In OFDM the data of a communication channel is broken into a large number of sub-carriers with each subcarrier modulated at a lower symbol rate than that of the channel, but the total rate of all the sub-carriers is equal to that of the conventionally modulated channel. In the case of a WDM optical network, each communication channel comprises a narrow defined wavelength range so that the sub-carriers fit within that wavelength range. OFDM has the advantage that its signals can manage severe channel conditions.
Notwithstanding techniques, such as OFDM, which can improve the performance of an optical network, the enthusiasm for ever-increasing optical performance overlooks a potential problem. The eventual aging and inevitable deterioration in performance of these networks have not been considered to any extent. Of course, one way of dealing with a declining network is to simply replace it. But this is a very expensive investment and a time-consuming operation.
An alternative is to keep the declining network in operation. In optical network design, the quality of performance is balanced against the costs of such performance as in any actual system. That is, among other considerations, the links of a network are designed with a particular bandwidth in mind. The definition or the shape of an optical signal naturally deteriorates as the signal travels along an optical fiber and as the bandwidth (i.e., the bit transfer rate) rises, the definition of the optical signal deteriorates faster. Thus the network, depending upon the signal bandwidth, must place optical amplifiers at locations along the link to regenerate the signals before they deteriorate beyond recognition. As network components age, optical performance naturally falls. Steps must be taken to ensure that the network operates at its maximum capacity despite the declining performance.
The present invention is directed generally toward adapting optical networks for declining optical performance and, in particular, toward adapting OFDM optical networks toward such decline.